Mobile communication systems, such as those mountable to a vehicle or manually deployable for terrestrial applications, customarily employ a vertical monopole antenna having a substantially omnidirectional radiation/sensitivity pattern. Preferably such antennas are both lightweight and compact, for ease of transport and attachment to an attendant transceiver housing. Because the effective electrical length of the monopole element is usually an appreciable fraction of a wavelength of the radiated wave (typically on the order of one-quarter to one-half a wavelength), geometries other than a straight wire have been proposed in order to reduce the actual physical length of the antenna structure to a manageable size. For example, each of the U.S. Pat. No. to Henderson, 4,087,820, Zandbergen, U.S. Pat. No. 4,435,716 and Eroncig, U.S. Pat. No. 4,097,867 describes a quarter wavelength monopole structure having a generally helically configured design for use with a citizens band radio. The helical monopole described in the Henderson patent is mounted within a variable length cylindrical tube or mast, while the conical helix antennas described by Zandbergen and Eroncig have a varying diameter along the longitudinal axis of the antenna. German Offenlegungsschrift No. 1813292 also discloses a conical helix antenna structure similar to that described by Zandbergen, formed of a spirally wound conductor that is fed from the wide diameter portion of the spiral.
Now although helical antenna structures of the type described in the above-referenced literature may be useful in mobile communication systems such as citizens band radios, they still possess substantial physical size that makes them readily visually detectable. For certain mobile or field deployable applications, the antenna must possess substantial low observability characteristics. For example, high performance aircraft customarily employ radiator elements that are embedded in or conformal with the airframe. In harsh terrestrial environments, such low observability characteristics particularly serve to minimize potential discovery by a variety of hostile or system defeating threats. Thus, the successful deployment of a radio-linked command and control system often depends not only upon its antenna's functionality as a preferably broadband electromagnetic wave interface device, but also upon the antenna's ability to perform such functionality, while still complying with minimum hardware requirements of the remainder of the system.
Unfortunately, conventional quarter-wavelength or longer monopole antennas, such as those described above, have a substantial dimension along the direction of deployment (often a length of several feet or more), so that they are not considered to possess low observability characteristics. In addition, where it is desired to extend the antenna coverage of a conventional monopole over a wide bandwidth, it is necessary to incorporate a plurality of switchably inserted or stepped tuning networks in circuit with the antenna, something which packaging constraints on a miniaturized transceiver package from which the antenna is deployed may prohibit.